1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus equipped with an ink sheet cartridge.
2. Background Information
Also, a thermal transfer printer equipped with an ink sheet cartridge is known as an example of an image forming apparatus. Such thermal transfer printer generally uses an ink cartridge, which is installed within a chassis of the thermal transfer printer with an engagement mechanism. As shown in FIGS. 14-16, the ink sheet cartridge 102 has a take-up portion 102a, a feed portion 102b, and a linking portion 102c. As shown in FIG. 14, a take-up bobbin 102d is rotatably disposed inside the take-up portion 102a of the ink sheet cartridge 102. A feed bobbin 102e is rotatably disposed inside the feed portion 102b of the ink sheet cartridge 102. A recess that engages an engagement mechanism is provided to the linking portion 102c of the ink sheet cartridge 102.
As shown in FIG. 14, the take-up bobbin 102d has one end portion 102g, another end portion 102h, and a shaft portion 102i. The feed bobbin 102e has one end portion 102j, another end portion 102k, and a shaft portion 102l. Compression coil springs 102m and 102n are attached to the end portion 102h of the take-up bobbin 102d and the end portion 102k of the feed bobbin 102e, respectively. These compression coil springs 102m and 102n bias the take-up bobbin 102d and the feed bobbin 102e in the direction of the take-up reel 103 and the feed reel 104 (the direction of arrow K2 in FIG. 14), respectively.
As shown in FIG. 11, a recess 102p having through grooves 102o is provided to the end portion 102g of the take-up bobbin 102d. A recess having three grooves just like those of the end portion 102g of the take-up bobbin 102d is provided to the end portion 102j of the feed bobbin 102e. As shown in FIG. 14, a film 102q is wound around the shaft portion 102i of the take-up bobbin 102d and the shaft portion 102l of the feed bobbin 102e. 
As shown in FIGS. 11 and 12, the take-up reel 103 includes a metal rotary shaft 103a, a plastic take-up bobbin engagement portion 103b attached to one end of the rotary shaft 103a, a torque limiter 103c attached to the other end of the rotary shaft 103a, compression coil springs 103d and 103e, metal retaining snap rings 103f and 103g, and position limiting snap rings 103h and 103i. As shown in FIG. 12, this rotary shaft 103a is rotatably attached to the side surface of the chassis by the bearing member 106, and its movement in the axial direction is limited by the position limiting snap rings 103h and 103i. 
As shown in FIG. 12, the take-up bobbin engagement portion 103b of the take-up reel 103 is disposed so as to protrude inward from the side surface of the chassis. This take-up bobbin engagement portion 103b is constituted so as to be capable of moving along the rotary shaft 103a in the direction of the arrow M2 in FIG. 12 against the biasing force of the compression coil spring 103d. As shown in FIG. 11, the take-up bobbin engagement portion 103b also has protruding ribs 103j that engage the grooves 102o of the end portion 102g of the take-up bobbin 102d. 
As shown in FIGS. 11 and 12, the torque limiter 103c has a gear 103k that is rotated by drive force from the motor, a felt portion 103l in contact with the gear 103k, and a circular plate 103m to which the felt portion 103l is attached. The compression coil spring 103d is disposed between the take-up bobbin engagement portion 103b and the position limiting snap ring 103h so that the take-up bobbin engagement portion 103b is biased toward the take-up bobbin 102d of the ink sheet cartridge 102. The compression coil spring 103e is disposed between the plate 103m and the retaining snap ring 103g so that the plate 103m is biased toward the gear 103k via the felt portion 103l. 
As shown in FIG. 13, the feed reel 104 includes a metal rotary shaft 104a, a plastic feed bobbin engagement portion 104b attached to one end of the rotary shaft 104a, a compression coil spring 104c, a metal retaining snap ring 104d, and position limiting snap rings 104e and 104f. This rotary shaft 104a is rotatably attached to the side surface of the chassis by the bearing member 107, and its position in the axial direction is limited by the position limiting snap rings 104e and 104f. 
The feed bobbin engagement portion 104b of the feed reel 104 is disposed so as to protrude inward from the side surface of the chassis. This feed bobbin engagement portion 104b is constituted so as to be capable of moving along the rotary shaft 104a in the direction of the arrow M2 in FIG. 13 against the biasing force of the compression coil spring 104c. As shown in FIG. 14, the plastic feed bobbin engagement portion 104b is disposed so as to engage the end portion 102j of the feed bobbin 102e. Also, the compression coil spring 104c is disposed between the feed bobbin engagement portion 104b and the position limiting snap ring 104e, so that the feed bobbin engagement portion 104b is biased toward the feed bobbin 102e of the ink sheet cartridge 102.
Next, the manner in which paper is fed and the film 102q is taken up in a conventional thermal transfer printer will be described through reference to FIGS. 11 and 12. During the paper feed operation, the torque from the motor is not transmitted to the gear 103k of the torque limiter 103c. Therefore, the take-up reel 103 does not rotate.
On the other hand, during the paper discharge operation (that is, during printing), the torque of the motor is transmitted to the gear 103k of the torque limiter 103c. As shown in FIG. 12, at this point, the plate 103m of the torque limiter 103c is biased toward the gear 103k by the compression coil spring 103e, so the drive force transmitted to the gear 103k is transmitted through the felt portion 103l to the plate 103m. As a result, the gear 103k and the plate 103m rotate integrally in the direction of the arrow F2 in FIG. 11.
The take-up bobbin engagement portion 103b is attached to the rotary shaft 103a so as to rotate integrally therewith. The rotary shaft 103a rotates integrally with the plate 103m of the torque limiter 103c. Thus, when the drive force from the motor is transmitted to the gear 103k of the torque limiter 103c, the torque limiter 103c rotates in the direction of the arrow F2 in FIG. 11, whereupon the take-up bobbin engagement portion 103b also rotates in the direction of the arrow F2. As a result, the take-up bobbin 102d engaged with the take-up bobbin engagement portion 103b also rotates in the direction of the arrow F2, causing the film 102q wound around the shaft portion 102i of the take-up bobbin 102d to be taken up. Also, when the film 102q is taken up, the feed bobbin 102e around which the film 102q is wound also rotates, and the feed reel 104 engaged with the feed bobbin 102e also rotates.
In this case, when a load torque that exceeds a predetermined torque is applied to the take-up bobbin engagement portion 103b, the felt portion 103l attached to the plate 103m as shown in FIG. 12 slips with respect to the gear 103k. Thus, even when the gear 103k rotates, the plate 103m does not. Consequently, when a load torque that exceeds a specific torque is applied to the take-up bobbin engagement portion 103b, the torque is not transmitted to the rotary shaft 103a, and the rotary shaft 103a does not rotate.
Next, the installation and ejection of the ink sheet cartridge 102 will be described through reference to FIGS. 14 to 16. First, in the installation of the ink sheet cartridge 102, the ink sheet cartridge 102 is moved from the state shown in FIG. 14 in the direction of the arrow K2. As a result, as shown in FIG. 15, one end portion 102g of the take-up bobbin 102d and one end portion 102j of the feed bobbin 102e inside the ink sheet cartridge 102 are engaged with the take-up bobbin engagement portion 103b of the take-up reel 103 and the feed bobbin engagement portion 104b of the feed reel 104, respectively.
Then, the take-up bobbin engagement portion 103b of the take-up reel 103 and the feed bobbin engagement portion 104b of the feed reel 104 are moved by the distance L2 (ejection stroke) in the direction of the arrow M2 shown in FIG. 15 against the biasing force of the compression coil springs 103d and 104c. Accordingly, the compression coil springs 103d and 104c are compressed by the amount L2. Then, the installation of the ink sheet cartridge 102 is completed by engaging an engagement portion of the engagement mechanism with the recess 102f of the linking portion 102c of the ink sheet cartridge 102.
In this installed state, the take-up bobbin 102d and the feed bobbin 102e are respectively biased toward the take-up reel 103 and the feed reel 104 (the direction of the arrow M2 in FIG. 15) by the compression coil spring 102m attached to the end portion 102h of the take-up bobbin 102d and the compression coil spring 102n attached to the end portion 102k of the feed bobbin 102e. Accordingly, the take-up bobbin 102d and the feed bobbin 102e are securely engaged with the take-up reel 103 and the feed reel 104, respectively.
Next, ejection of the ink sheet cartridge 102 is performed by disengaging the ink sheet cartridge from the engagement mechanism. As a result, the compression coil springs 103d and 104c of the take-up reel 103 and the feed reel 104 try to expand by the amount they were compressed (the ejection stroke L2 shown in FIG. 15). Accordingly, an ejection force is applied in the direction of the arrow N2 shown in FIG. 17 to the ink sheet cartridge 102. As a result, as shown in FIG. 16, the ink sheet cartridge 102 is moved in the eject direction by the ejection stroke L2 in FIG. 15 from the chassis.
With the conventional thermal transfer printer shown in FIGS. 11-16, the ejection force and ejection stroke (L2 in FIG. 15) at the time of ejection of the ink sheet cartridge 102 is obtained by compressing the compression coil spring 103d of the take-up reel 103 and the compression coil spring 104c of the feed reel 104. Also, the take-up bobbin 102d and the feed bobbin 102e are securely engaged with the take-up reel 103 and the feed reel 104 with the compression coil springs 102m and 102n, which are respectively attached to the end portions 102h and 102k of the take-up bobbin 102d inside the ink sheet cartridge 102. In other words, in the conventional thermal transfer printer shown in FIGS. 11-16, the compression coil spring 103d of the take-up reel 103 and the compression coil spring 104c of the feed reel 104, which are used to eject the ink sheet cartridge 102, are provided separately from the compression coil springs 102m and 102n, which are used to securely engage the take-up bobbin 102d and the feed bobbin 102e with the take-up reel 103 and the feed reel 104. Such construction requires a large number of parts.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved thermal transfer printer that overcomes the problems of the present invention. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.